The post-translational modification of proteins by ubiquitin-like molecules (ubls) is an important regulatory process within cells, playing key roles in controlling many biological processes including cell division, cell signaling and the immune response. Ubls are small proteins that are covalently attached to a lysine on a target protein via an isopeptide linkage with a C-terminal glycine of the ubl. The ubiquitin-like molecule alters the molecular surface of the target protein and can affect such properties as protein-protein interactions, enzymatic activity, stability and cellular localization of the target.
Ubiquitin and other ubls are activated by a specific E1 enzyme which catalyzes the formation of an acyl-adenylate intermediate with the C-terminal glycine of the ubl. The activated ubl molecule is then transferred to the catalytic cysteine residue within the E1 enzyme through formation of a thioester bond intermediate. The E1-ubl intermediate and an E2 associate, resulting in a thioester exchange wherein the ubl is transferred to the active site cysteine of the E2. The ubl is then conjugated to the target protein, either directly or in conjunction with an E3 ligase, through isopeptide bond formation with the amino group of a lysine side chain in the target protein.
The biological consequence of ubl modification depends on the target in question. Ubiquitin is the best characterized of the ubls and a consequence of modification by ubiquitination is the degradation of poly-ubiquitinated proteins by the 26S proteasome. Ubiquitin is conjugated to its target proteins through an enzymatic cascade involving its specific E1 activating enzyme, Uba1 (ubiquitin activating enzyme, UAE), a conjugating enzyme from the family of E2s, and a ubiquitin ligase from either the RING or HECT classes of E3s. See, Huang et al., Oncogene, 23:1958-71 (2004). Target specificity is controlled by the particular combination of E2 and E3 protein, with >40 E2s and >100 E3s being known at present. In addition to ubiquitin, there are at least 10 ubiquitin-like proteins, each believed to be activated by a specific E1 activating enzyme and processed through similar but distinct downstream conjugation pathways. Other ubls for which E1 activating enzymes have been identified include Nedd8 (APPBP1-Uba3), ISG15 (UBE1L) and the SUMO family (Aos1-Uba2).
The ubl Nedd8 is activated by the heterodimer Nedd8-activating enzyme (APPBP1-Uba3) (NAE) and is transferred to a single E2 (Ubc12), ultimately resulting in ligation to cullin proteins. The function of neddylation is the activation of cullin-based ubiquitin ligases involved in the ubiquitination and hence turnover of many cell cycle and cell signaling proteins, including p27 and I-κB. See Pan et al., Oncogene. 23:1985-97 (2004). The ubl SUMO is activated by the heterodimer sumo activating enzyme (Aos1-Uba2) (SAE) and is transferred to a single E2 (Ubc9), followed by coordination with multiple E3 ligases, ultimately resulting in sumoylation of target proteins. Sumo modification can affect the cellular localization of target proteins and proteins modified by SUMO family members are involved in nuclear transport, signal transduction and the stress response. See Seeler and Dejean, Nat Rev Mol Cell Biol. 4:690-9, (2003). The function of sumoylation includes activation of cell signaling pathways (e.g., cytokine, WNT, growth factor, and steroid hormone signaling) involved in transcription regulation; as well as pathways involved in control of genomic integrity (e.g., DNA replication, response to DNA damage, recombination and repair). See Muller et al, Oncogene. 23:1998-2006, (2004). There are other ubls (e.g., ISG15, FAT10, Apg12p) for which the biological functions are still under investigation.
A particular pathway of importance which is regulated via E1 activating enzyme activities is the ubiquitin-proteasome pathway (UPP). As discussed above, the enzymes UAE and NAE regulate the UPP at two different steps in the ubiquitination cascade. UAE activates ubiquitin in the first step of the cascade, while NAE, via activation of Nedd8, is responsible for the activation of the cullin based ligases, which in turn are required for the final transfer of ubiquitin to certain target proteins A functional UPP pathway is required for normal cell maintenance. The UPP plays a central role in the turnover of many key regulatory proteins involved in transcription, cell cycle progression and apoptosis, all of which are important in disease states, including tumor cells. See, e.g., King et al., Science 274: 1652-1659 (1996); Vorhees et al., Clin. Cancer Res., 9: 6316-6325 (2003); and Adams et al., Nat. Rev. Cancer, 4: 349-360 (2004). Proliferating cells are particularly sensitive to inhibition of the UPP. See, Drexler, Proc. Natl. Acad. Sci., USA 94: 855-860 (1977). The role of the UPP pathway in oncogenesis has led to the investigation of proteasome inhibition as a potential anticancer therapy. For example, modulation of the UPP pathway by inhibition of the 26S proteasome by VELCADE® (bortezomib) has proven to be an effective treatment in certain cancers and is approved for the treatment of multiple myeloma and mantle cell lymphoma patients who have received at least one prior therapy. Examples of proteins whose levels are controlled by cullin-based ubiquitin ligases which are downstream of NAE and UAE activity include the CDK inhibitor p27Kip1 and the inhibitor of NFκB, IκB. See, Podust et al., Proc. Natl. Acad. Sci., 97: 4579-4584 (2000), and Read et al., Mol. Cell Biol., 20: 2326-2333 (2000). Inhibition of the degradation of p27 is expected to block the progression of cells through the G1 and S phases of the cell cycle. Interfering with the degradation of IκB should prevent the nuclear localization of NF-κB, transcription of various NF-κB-dependent genes associated with the malignant phenotype, and resistance to standard cytotoxic therapies. Additionally, NF-κB plays a key role in the expression of a number of pro-inflammatory mediators, implicating a role for such inhibitors in inflammatory diseases. Furthermore, inhibition of UPP has been implicated as a useful target for additional therapeutics, such as inflammatory disorders, including, e.g., rheumatoid arthritis, asthma, multiple sclerosis, psoriasis and reperfusion injury; neurodegenerative disorders, including e.g., Parkinson's disease, Alzheimer's disease, triplet repeat disorders; neuropathic pain; ischemic disorders, e.g., stroke, infarction, kidney disorders; and cachexia. See, e.g., Elliott and Ross, Am. J. Clin. Pathol., 116:637-46 (2001); Elliott et al., J. Mol. Med., 81:235-45 (2003); Tarlac and Storey, J. Neurosci. Res. 74: 406-416 (2003); Mori et al., Neuropath. Appl. Neurobiol., 31: 53-61 (2005); Manning, Curr. Pain Headache Rep., 8: 192-8 (2004); Dawson and Dawson, Science, 302: 819-822 (2003); Kukan, J. Physiol. Pharmacol., 55: 3-15 (2004); Wojcik and DiNapoli, Stroke, 35:1506-18 (2004); Lazarus et al., Am J Physiol., 27:E332-41 (1999).
Targeting E1 activating enzymes provides a unique opportunity to interfere with a variety of biochemical pathways important for maintaining the integrity of cell division and cell signaling. E1 activating enzymes function at the first step of ubl conjugation pathways; thus, inhibition of an E1 activating enzyme will specifically modulate the downstream biological consequences of the ubl modification. As such, inhibition of these activating enzymes, and the resultant inhibition of downstream effects of ubl-conjugation, represents a method of interfering with the integrity of cell division, cell signaling, and several aspects of cellular physiology which are important for disease mechanisms. Thus, E1 enzymes such as UAE, NAE, and SAE, as regulators of diverse cellular functions, are potentially important therapeutic targets for the identification of novel approaches to treatment of diseases and disorders.